Companies involved in the packaging of foods have traditionally had two main concerns: 1) packaging the food item in such a manner that it has a long shelf life, and 2) avoiding package leakage. Shelf life has always been important and is becoming more so with the increased sale of highly processed and cooked foods which spoil relatively quickly. Increasing shelf life has numerous advantages, including ease of distribution, improved product appearance, and most importantly safety, by maintaining the freshness of the product for an extended period. Product leakage relates to the leakage of fluids, primarily from meat products. Products which leak obviously present aesthetic and handling problems.
A recently developed technology for increasing shelf life and avoiding leakage is that of modified atmosphere packaging, or MAP, also known as controlled atmosphere packaging, or CAP. MAP packaging involves hermetically sealing the product with an appropriate type of shrink barrier film, meaning a film that is impervious to selected gases. Many types of co-extruded multilayer shrink barrier films are commercially available. The film is shaped to form a package with the product inside, and the air is flushed out of the package and replaced with a gas or mixture of gases that is inert to the product and that retards spoilage. The modified atmosphere in the package increases shelf life, and the barrier film and hermetic sealing ensure that the modified atmosphere will not change significantly over time. The hermetic seal is, of course, leakproof.
Prior art MAP machines typically are of the form fill type, meaning the film used to package the product is formed to receive the product and the product fills the film thus formed. Some form fill machines are horizontal, meaning the packages move through the machines horizontally, while others are vertical. The film is usually formed in the shape of a tube, and after the product enters the tube a longitudinal seal known as a lap seal is made, in horizontal machines, along the bottom of the package. In vertical machines, the lap seal is made along the back of the package; the term "bottom" as used herein will be understood to include the back of packages in vertical machines. Cross seals are then made at the front and rear of the package. Products packaged in this manner sometimes experience capillary leakage at the points where the cross seals and the longitudinal lap seal intersect, since sealing must take place through a multiplicity of film layers at these points. This capillary leakage can shorten the shelf life of the product and can result in an unsafe condition if product spoilage goes undetected. The intersections of cross seals and lap seal also tend to be unsightly.
In addition to poor sealing, lap sealing also leaves less room on the package for printing nutrition information, recipes, and the like. Government regulations mandate that a certain distance be maintained between printed material and seals; since a lap seal is in the center of the bottom of the package, essentially none of the bottom portion of the package can be used for printing. Also, since the lap seal is disposed on a major flat surface of the product, it is prone to catching during handling which can result in a tear or leak.
The aesthetic and printing space problems associated with lap seals in MAP systems are also present in other form-fill shrink film applications and in form-fill stretch and stretch/shrink film packaging systems. Stretch and stretch/shrink film systems have additional problems as well. Such systems usually package a product by making a lap seal and leaving large flaps of excess film at the front and rear of the product. These flaps of film are then folded around the product and heated to cause them to stick to the film on the bottom of the product. A hermetic seal is not usually sought in these applications; nevertheless, the package must still be leakproof. A large amount of excess film is used for the flaps to avoid leakage, increasing the expense of such systems, but leakage often continues to be a problem.
It has been found that heat seals are more effective if the film is preheated before sealing. However, prior art sealing machines have not had an effective or convenient way of preheating the film.